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Old 11th September, 2004, 09:31 AM
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mookydooky mookydooky is offline
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RAID levels and what they're good for:

RAID 0: Striping without parity. Striping is basically reading data consecutively off of multiple drives. Imagine moving blocks from one pile to the middle using one hand. The speed is limited by your hand speed. Now imagine having two piles of blocks and you can use both hands to move the blocks to the middle. You could theoretically do the work twice as fast. This concept can be be done over any number of drives. The risk of course, is drive failure and data loss and corruption. You're increasing the risk by a factor of X, where X is the number of drives.

RAID 1: Mirror. RAID 1 is the most basic of RAIDs. It's basic redundancy where all data is identically written to multiple drives. In the event of hardware failure, you're back up very quickly (some controllers even do it on the fly) because everything has been backed up already.

RAID 2-3-4: ECC (Error checking code) and parity. By adding extra bits to the end of data, it possible to both check for errors and also repair errors. RAID 2 allowed for on the fly reconstruction of data in the event of disk failure while at the same time, offering some performance gain by striping the data. This was done by dedicating disks for parity. RAID 2 however, tended to be very disk hungry and thus they came up with RAID 3. RAID 3 moved on to byte level data. Larger data chunks allow for better performance. It also used a better ECC scheme using XOR. RAID 3's basic problem was the dedicated parity drive. RAID 3's performance wasn't has good as it could be because of all the writes to the parity drive. RAID 4 is basically RAID 3 but the data size is much larger now. RAID 4 uses whole files instead of bytes. This allows really fast read speeds since multiple files could be accessed at once.

RAID 5-6: Striping and parity in one big package. Imagine RAID 3, but instead of a dedicated parity disk, where bottlenecks occur, you spead the ECC over multiple disks. Real world performance is close to RAID 0, but with parity using XOR ECC. In the event of a disk failure, RAID 5 can rebuild the data on the fly depending on the controller's capabilities. You can even hotswap disk and have the controller rebuild the drive using the parity bits on the other disks. Pretty fancy stuff. RAID 6 builds on RAID 5 by adding an addition parity set. With RAID 6, it's possible to have two drives go down the toilet and still be able to rebuild the data.
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